The white dwarf is a star in our space is quite common. Scientists call it the result of the evolution of stars, the final stage of development. There are two scenarios for modifying a stellar body, in one case the final stage is a neutron star, in the other a black hole. Dwarfs are the final evolutionary step. Around them there are planetary systems. Scientists were able to determine this by studying the samples enriched with metals.
Background
White dwarfs are stars that attracted the attention of astronomers in 1919. For the first time, such a celestial body was discovered by a scientist from the Netherlands, Maanen. For his time, the specialist made a rather atypical and unexpected discovery. The dwarf that he saw was like a star, but had non-standard small sizes. The spectrum, however, was as if it were a massive and large celestial body.
The reasons for this strange phenomenon have attracted scientists for quite a long time, so a lot of effort was made to study the structure of white dwarfs. The breakthrough came when they expressed and proved the assumption of the abundance of various metal structures in the atmosphere of a celestial body.
It is necessary to clarify that metals in astrophysics are all kinds of elements whose molecules are heavier than hydrogen and helium, and their chemical composition is more progressive than these two compounds. Helium, hydrogen, as scientists have been able to establish, is more widespread in our universe than any other substance. Based on this, it was decided to designate everything else as metals.
Theme development
Although for the first time white dwarfs, which were very different in size from the Sun, were noticed in the twenties, only half a century later, people revealed that the presence of metal structures in the stellar atmosphere is not a typical phenomenon. As it turned out, when included in the atmosphere, in addition to the two most common substances, heavier ones, they are displaced into the deeper layers. Heavy substances, being among the molecules of helium, hydrogen, should eventually move to the core of the star.
Several reasons were found for this process. The radius of the white dwarf is small, such stellar bodies are very compact - it is not for nothing that they got their name. On average, the radius is comparable to that of the earth, while the weight is similar to the weight of a star illuminating our planetary system. This ratio of dimensions and weight causes extremely large gravitational surface acceleration. Consequently, the deposition of heavy metals in a hydrogen and helium atmosphere occurs only a few Earth days after the molecule enters the total gas mass.
Features and Duration
Sometimes the characteristics of white dwarfs are such that the process of sedimentation of molecules of heavy substances can drag on for a long time. The most favorable options, from the point of view of an observer from the Earth, are processes that take millions, tens of millions of years. Nevertheless, such time intervals are extremely small in comparison with the duration of the existence of the stellar body itself.
The evolution of the white dwarf is such that most of the formations currently observed by man now number several hundred million Earth years. If you compare this with the slowest process of metal absorption by the core, the difference is more than significant. Consequently, the detection of metal in the atmosphere of a certain observed star allows us to conclude with confidence that initially the body did not have such an atmosphere composition, otherwise all metal inclusions would have disappeared long ago.
Theory and practice
The observations described above, as well as information collected over many decades about white dwarfs, neutron stars, black holes, suggested that the atmosphere receives metallic inclusions from external sources. Scientists first decided that this is the medium between the stars. The celestial body moves through such a substance, accretes the medium onto its surface, thereby enriching the atmosphere with heavy elements. But further observations showed that such a theory is untenable. As the experts specified, if the atmosphere changed in this way, the dwarf would mainly receive hydrogen from the outside, since the medium between the stars is formed in its bulk by hydrogen and helium molecules. Only a small percentage of the medium is heavy compounds.
If the theory formed from the initial observations of white dwarfs, neutron stars, black holes would justify itself, the dwarfs would consist of hydrogen as the lightest element. This would not allow the existence of even helium celestial bodies, because helium is heavier, which means that hydrogen accretion would completely hide it from the eyes of an external observer. Based on the presence of helium dwarfs, scientists came to the conclusion that the interstellar medium cannot serve as the only and even the main source of metals in the atmosphere of stellar bodies.
How to explain?
Scientists involved in black holes, white dwarfs in the 70s of the last century suggested that metallic inclusions can be explained by the fall of comets on the surface of a celestial body. True, at one time such ideas were considered too exotic and did not receive support. This was largely explained by the fact that people did not yet know about the availability of other planetary systems - only our “home” Solar was known.
A significant step forward in the study of black holes, white dwarfs was taken at the end of the next, eighth decade of the last century. Scientists have at their disposal especially powerful infrared devices for observing the depths of space, which made it possible to detect infrared radiation around one of the famous white dwarf astronomers. This was revealed precisely around the dwarf, whose atmosphere contained metallic inclusions.
Infrared radiation, which made it possible to estimate the temperature of the white dwarf, also informed scientists that the stellar body is surrounded by some substance capable of absorbing stellar radiation. This substance is heated to a specific temperature level lower than that of a star. This allows you to gradually redirect the absorbed energy. The radiation occurs in the infrared range.
Science moves forward
The spectra of the white dwarf have become the object of study of the advanced minds of the world of astronomers. As it turned out, quite extensive information about the features of celestial bodies can be obtained from them. Of particular interest were observations of stellar bodies with excess infrared radiation. Currently, it has been possible to identify about three dozen systems of this type. Their main percentage was studied using the powerful Spitzer telescope.
Scientists, observing the celestial bodies, found that the density of white dwarfs is significantly lower than this parameter, characteristic of giants. It was also revealed that excess infrared radiation is explained by the presence of disks formed by a specific substance capable of absorbing energy radiation. It then radiates energy, but in a different wavelength range.
The disks are located extremely close and to some extent affect the mass of white dwarfs (which cannot exceed the Chandrasekhar limit). The outer radius is called the clastic disk. It has been suggested that such was formed during the destruction of a certain body. On average, the radius is comparable in size to the sun.
If we pay attention to our planetary system, it becomes clear that relatively close to the "house" we can observe a similar example - these are the rings surrounding Saturn, the size of which is also comparable to the radius of our star. Over time, scientists have established that this feature is not the only one that makes dwarfs and Saturn related. For example, both the planet and stars have very thin disks, which are unusual for transparency when trying to shine through with light.
Conclusions and development of the theory
Since white dwarf rings are comparable to those surrounding Saturn, it has become possible to formulate new theories that explain the presence of metals in the atmosphere of these stars. Astronomers know that around Saturn the rings are formed by the tidal destruction of some bodies that are close enough to the planet to be affected by its gravitational field. In such a situation, the external body cannot maintain its own gravity, which leads to a violation of integrity.
About fifteen years ago, a new theory was introduced that explained the formation of white dwarf rings in a similar way. It was suggested that the dwarf was originally a star in the center of the planetary system. The celestial body evolves over time, which takes billions of years, swells, loses its shell, and this causes the formation of a dwarf that gradually cools. Incidentally, the color of white dwarfs is explained precisely by their temperature. For some, it is estimated at 200,000 K.
The system of planets during this evolution can survive, which leads to the expansion of the outer part of the system simultaneously with a decrease in the mass of the star. As a result, a large system of planets is formed. Planets, asteroids, and many other elements survive evolution.
What's next?
The progress of the system can lead to its instability. This leads to a stone bombardment of the surrounding planet space, and asteroids partially fly out of the system. Some of them, however, move into orbits, sooner or later appearing within the solar radius of the dwarf. Collision does not occur, but tidal forces lead to a violation of the integrity of the body. The accumulation of such asteroids takes on a form similar to the rings surrounding Saturn. Thus, a disk of debris is formed around the star. The density of the white dwarf (about 10 ^ 7 g / cm3) and its clastic disk differs significantly.
The described theory has become a fairly complete and logical explanation of a number of astronomical phenomena. By means of it, one can understand why the disks are compact, because the star cannot be surrounded by a disk all the time of its existence, the radius of which is comparable to the solar one, otherwise at first such disks would be inside her body.
Having explained the formation of the disks and their size, we can understand where a peculiar supply of metals comes from. It can appear on a stellar surface, contaminating the dwarf with metal molecules. The described theory, without contradicting the revealed indicators of the average density of white dwarfs (of the order of 10 ^ 7 g / cm3), proves why the metals are observed in the atmosphere of stars, why measurement of the chemical composition is possible by human means and why the distribution of elements is similar to which is characteristic of our planet and other studied objects.
Theories: is there any use?
The described idea was widely used as a basis for explaining why the shells of stars are contaminated with metals, why clastic disks appeared. In addition, it follows that there is a planetary system around the dwarf. There is little surprising in this conclusion, because humanity has established that most of the stars have their own planetary systems. This is characteristic both in that they are similar to the Sun, and in that they are much larger in size - namely, white dwarfs are formed from them.
Themes are not exhausted.
Even if we consider the theory described above generally accepted and proven, some questions for astronomers to this day remain open. Of particular interest is the specificity of the transfer of matter between disks and the surface of a celestial body. As some have suggested, this is due to radiation. Theories calling for a description of substance transfer in this way are based on the Poynting-Robertson effect. This phenomenon, under the influence of which particles slowly move in an orbit around a young star, gradually spiraling to the center and disappearing into the celestial body. Presumably, this effect should manifest itself on the detrital disks surrounding the stars, that is, the molecules that are present in the disks sooner or later find themselves in exceptional proximity to the dwarf. Solids are subject to evaporation, gas is formed - such in the form of disks was fixed around several observed dwarfs. Sooner or later, the gas reaches the surface of the dwarf, transferring metals here.
The revealed facts are estimated by astronomers as a significant contribution to science, since they suggest how the planets are formed. This is important because research facilities that attract specialists are often not available. For example, planets that revolve around the size of stars exceeding the Sun can rarely be studied - it is too difficult at the technical level that is accessible to our civilization. Instead, people got the opportunity to study planetary systems after turning stars into dwarfs. If it is possible to develop in this direction, it will certainly be possible to reveal new data on the presence of planetary systems and their distinctive characteristics.
White dwarfs, in the atmosphere of which metals are detected, make it possible to get an idea of the chemical composition of comets and other cosmic bodies. In fact, scientists simply do not have another way to evaluate the composition. For example, studying giant planets, one can only get an idea of the outer layer, but there is no reliable information about the inner content. This also applies to our "home" system, since the chemical composition can be studied only from that celestial body that fell to the surface of the Earth or from where the apparatus for research was able to land.
How is everything going?
Sooner or later, our planetary system will also become the "home" of a white dwarf. According to scientists, the stellar core has a limited volume of matter for energy, and sooner or later, fusion reactions are exhausted. Gas decreases in volumes, the density rises to tons per cubic centimeter, while in the outer layers the reaction continues. A star expands, becomes a red giant, the radius of which is comparable to hundreds of stars equal to the Sun. When the outer shell stops "burning", the substance disperses in space for 100,000 years, which is accompanied by the formation of a nebula.
The core of the star, freed from the shell, lowers the temperature, which leads to the formation of a white dwarf. In fact, such a star is a high-density gas. In science, dwarfs are often called degenerate celestial bodies. If our luminary shrank and its radius would count only a few thousand kilometers, but the weight would be completely preserved, then there would also be a white dwarf.
Features and technical highlights
The considered type of a cosmic body is capable of glowing, but this process is explained by other mechanisms other than thermonuclear reactions. The glow is called residual, it is explained by a decrease in temperature. The dwarf is formed by a substance whose ions are sometimes colder than 15,000 K. The elements are characterized by vibrational movements. Gradually, the celestial body becomes crystalline, its glow weakens, and the dwarf evolves into brown.
Scientists have identified a mass limit for such a celestial body - up to 1.4 times the weight of the Sun, but not more than this border. If the mass exceeds this limit, the star cannot exist. This is due to the pressure of the substance in a compressed state - it is less than the gravitational attraction compressing the substance. A very strong compression occurs, which leads to the appearance of neutrons, the substance is neutronized.
The compression process can lead to degeneration. In this case, a neutron star is formed. The second option is to continue compression, sooner or later leading to an explosion.
General parameters and features
The bolometric luminosity of the considered category of celestial bodies relative to the characteristic of the Sun is less than about ten thousand times. The dwarf radius is less than a hundred times smaller than the sun, while the weight is comparable to the characteristic main star of our planetary system. To determine the mass boundary for the dwarf, the Chandrasekhar limit was calculated. When it is exceeded, the dwarf evolves into another form of the celestial body. The star’s photosphere on average consists of dense matter estimated at 105–109 g / cm3. Compared to the main stellar sequence, it is denser about a million times.
Some astronomers believe that only 3% of all stars in the galaxy are white dwarfs, and some are convinced that every tenth belongs to this class. Estimates differ so much about the reason for the difficulty of observing celestial bodies - they are remote from our planet and shine too weakly.
Stories and Names
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